<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div id="app">
    <ul>
      <li v-for="item in arr" :key="item">{{item}}</li>

    </ul>
    <button @click="add">button</button>

  </div>

</body>
<script src="../dist/vue.js"></script>
<script>
  let id = 4
  const vm = new Vue({
    el: '#app',
    data: {
      msg: 'Hello Word',
      // arr: [1, 2, 3]
    },
    methods: {
      add() {
        // this.arr.splice(0, 0, id++)
      }
    }
  })

</script>

</html>
